1. The Field of the Invention
The invention relates to apparatus, systems, and methods for generating hydrogen from a chemical hydride. Specifically, the invention relates to apparatus, systems, and methods for generating hydrogen by a self-regulated chemical reaction between water and a chemical hydride.
2. The Relevant Art
Various energy sources are used to fuel today's society. Fossil fuels such as coal, oil, and gas are some of the most commonly used fuels due to the comparatively large quantities available and minimal expense required to locate, collect, and refine the fossil fuels into usable energy sources. Alternative energy sources are available. Some of the alternative energy sources are readily available; however, the cost to generate, collect, or refine the alternative energy sources traditionally outweighs the benefits gained from the alternative energy sources.
Hydrogen is a plentiful alternative energy source; however, hydrogen generally exists as a molecule combined with one or more other elements. The additional elements add mass and may prevent the hydrogen from being a usable energy source. As a result, pure hydrogen is desired for use as an energy source. Pure hydrogen comprises free hydrogen atoms or molecules comprising only hydrogen atoms. Producing pure hydrogen using conventional methods requires expensive, bulky, and heavy storage containers.
In the prior art, pure hydrogen is generated by a chemical reaction which produces either free hydrogen atoms or hydrogen molecules. One such chemical reaction occurs between water, H2O, and chemical hydrides. Chemical hydrides are molecules comprising hydrogen and one or more alkali or alkali-earth metals. Examples of chemical hydrides include lithium hydride (LiH), lithium tetrahydridoaluminate (LiAlH4), lithium tetrahydridoborate (LiBH4), sodium hydride (NaH), sodium tetrahydridoaluminate (NaAlH4), sodium tetrahydridoborate (NaBH4), and the like. The chemical hydrides produce large quantities of pure hydrogen when reacted with water, as shown in reaction 1.LiAlH4(s)+2H2O(g,l)→LiAlO2(s)+4H2(g)  (1)
Recently, the interest in hydrogen generation from chemical hydrides has increased, due to the development of lightweight, compact Proton Exchange Membrane (PEM) fuel cells. One by-product of the PEM fuel cells is water that can be used to produce pure hydrogen from chemical hydrides for fuelling the PEM fuel cell. The combination of PEM fuel cells with a chemical hydride hydrogen generator offers advantages over other energy storage devices in terms of gravimetric and volumetric energy density, as shown in Table 1.
TABLE 1Density Comparison for Various Energy Storage Devices producing 220 W for12 hrsSourceTotalSpecificSpecificRequiredEnergyFuelTotalEnergyConversionConv.Energy(W-h/WtSystemWt.(W-h/Energy SourceTypeEff.(W-h)kg)(Kg)Wt (kg)(kg)kg)LiBH4 (waterPEMFC50%5,28012,3000.432.933.36785recycling)LiH (water recycling)PEMFC50%5,2808,5000.623.093.71712LiAlH4 (waterPEMFC50%5,2807,1000.743.19 3.93672recycling)NaH (waterPEMFC50%5,2802,8001.894.095.98441recycling)GasolineReformer/30%8,80012,3000.725.466.18427PEMFCDieselReformer/30%8,8009,9000.895.536.42411PEMFCPropaneReformer/30%8,80012,9000.686.006.68395PEMFCNatural GasReformer/30%8,80011,9000.746.066.80388PEMFCGasolineICE15%17,60012,3001.435.466.89383NaBH4 (20%PEMFC50%5,2801,4203.724.077.79339hydride/watersolution)Zinc-Air RefuelableDirect60%4,40040011.000.0011.00240BatteryMethanol (25%DMFC30%8,8001,3756.405.1511.55229methanol/watersolution)Magnesium HydridePEMFC50%5,2801,2004.4011.3915.79167CompressedPEMFC50%5,28033,3000.1623.6023.76113Hydrogen (4.5 KPSI)Li-ion BatteryDirect90%2,93312024.440.0024.44108(rechargeable)NiMH BatteryDirect90%2,9337041.900.0041.9063(rechargeable)Lead Acid BatteryDirect90%2,9334073.330.0073.3336(rechargeable)Notes:1) PEMFC weight based on 85 W/kg. Reformer weight equals PEMFC weight. ICE weight based on 45 W/kg. H2 cylinder weight based on 130 kg tank/kg fuel. Methane/Propane cylinder weight based on 1.2 kg tank/kg fuel. Tank weight for other liquid sources based on 0.4 kg tank/kg fuel. Tank weight for other solid sources based on 0.8 kg tank/kg fuel.2) The energy densities of the chemical hydrides were calculated assuming that water produced in the PEMFC is recycled to react with the chemical hydride.3) The energy densities of all other sources are generally accepted values from the literature.4) The DMFC was assumed to be 30% efficient. The PEMFC was assumed to be 50% efficient. The hydrogen reformer was assumed to be 60% efficient. A small ICE was assumed to be 15% efficient. 90% of the energy in the battery was assumed to be recoverable.
Unfortunately, the prior art has encountered unresolved problems producing pure hydrogen from chemical water/hydride reactions. Specifically, conventional systems, methods, and apparatus have not successfully controlled the chemical reaction between the water and the chemical hydride without adversely affecting the gravimetric and volumetric energy density of the overall system.
The chemical reaction between water and chemical hydrides is very severe and highly exothermic. The combination of the water and the chemical hydride must be precisely controlled to prevent a runaway reaction or an explosion. Many attempts have been made to properly control the reaction while still preserving the gravimetric and volumetric energy density provided by the chemical hydrides.
For example, the chemical hydride may be mixed with a stable, non-reactive organic liquid that allows the chemical hydride to be safely stored until it is needed. To produce hydrogen at a desired rate the chemical hydride/organic liquid mixture is brought in contact with a catalyst. In another example, a nonflammable solution of 20% NaBH4, 75% water and 5% NaOH, produces hydrogen when brought in contact with a catalyst. While these examples may successfully control the water/chemical hydride reaction, the stabilizing materials and catalysts are non-reactive materials that add weight and do not store energy. Consequently, the energy density of the storage device is reduced.
In another prior art approach, impermeable polyethylene balls encapsulate a chemical hydride. The ping-pong ball size spheres are stored in water until hydrogen is required, at which point a special cutting system slices the ball in half, exposing the chemical hydride to the water. Unfortunately, this approach includes many moving mechanical parts that are subject to malfunction, increase the weight of the system, and increase the cost of the hydrogen generator.
Finally, certain prior art techniques separate water from the chemical hydride using a membrane. Generally, the membranes pass water because of a difference in water pressure across the membrane. Water pressure on the side of the membrane opposite the chemical hydride pushes the water through the membrane. Other membranes utilize capillary action to transport water from one side of the membrane to the other. Consequently, a water supply must be provided that supplies water to the water side of the membrane to be transported by capillary action to the chemical hydride side of the membrane.
Unfortunately, prior art attempts to control the chemical reaction of water and chemical hydrides using membranes have limitations. Specifically, the membranes require that a water supply be constantly maintained such that sufficient water and/or water pressure is available on the water side of the membrane. As a result, the water supply increases the overall weight and complexity of the system.
Furthermore, in order for the membranes to properly function, the water must be in a liquid form. The liquid water increases the weight of the system and limits the environments in which the conventional hydrogen generator may be operated, because the water must stay at a temperature between the freezing and boiling points of the water. In addition, the membranes generally have pores that allow the water to pass. These pores may also allow hydrogen to pass back through the membrane toward the water side of the membrane which complicates the collection of the hydrogen.
Accordingly, what is needed is an improved apparatus, system, and method that overcomes the problems and disadvantages of the prior art. The apparatus, system, and method should not include a catalyst or stabilizing material to control the chemical water/hydride reaction. In particular, the apparatus, system, and method should control a chemical reaction between water and a chemical hydride using a membrane without relying on a water pressure differential across the membrane. The membrane should allow substantially only water to pass. In addition, the apparatus, system, and method should control a chemical reaction between water and a chemical hydride using a membrane that functions whether the water is in a liquid state or a gaseous state. Furthermore, the apparatus, system, and method should be compact, lightweight, simple, and maximize the gravimetric and volumetric energy density of the chemical hydride. Such an apparatus, system, and method are disclosed herein.